Fri 8/23

Two MIR color selected evolutionary studies, on hot dust, and radio AGN.


Hot-Dust (690K) Luminosity Density and its Evolution in the last 7.5Gyr

Hugo Messias, Bahram Mobasher, José Manuel Afonso
(Submitted on 21 Aug 2013)

[Abridged] We study the contribution of hot-dust to the luminosity density of galaxies and its evolution with cosmic time. Using the Spitzer-IRAC data in the COSMOS field, we estimate the contribution from hot-dust at rest-frame 4.2um (from ~0 < z < ~0.2 up to ~0.5 < z < ~0.9). This wavelength corresponds to black-body temperature of ~690K. The contribution due to stellar emission is estimated from the rest-frame 1.6um luminosity (assumed to result from stellar emission alone) and subtracted from the mid-infrared luminosity of galaxies to measure hot-dust emission. In order to attempt the study of the 3.3um-PAH feature, we use the rest-frame 4.2um to infer the hot-dust flux at 3.3um. This study is performed for different spectral types of galaxies: early-type, late-type, starburst, and IR-selected AGN. We find that: (a) the decrease of the hot-dust luminosity density since ~0.5 < z < ~1 is steeper (by at least ~0.5dex) compared to that of the cold-dust, giving support to the scenario where galaxy obscuration increases with redshift, as already proposed in the literature; (b) hot-dust and PAH emission evolution seems to be correlated with stellar mass, where rest-frame 1.6um luminous non-AGN galaxies (i.e., massive systems) show a stronger decrement (with decreasing redshift) in hot-dust and PAH emission than the less luminous (less massive) non-AGN galaxies; (c) despite comprising < ~3% of the total sample, AGN contribute as much as a third to the hot-dust luminosity density at z < 1 and clearly dominate the bright-end of the total hot-dust Luminosity Density Function at ~0.5 < z < ~0.9; (d) the average dust-to-total luminosity ratio increases with redshift, while PAH-to-total luminosity ratio remains fairly constant; (e) at M_1.6 > -25, the dust-to-total and PAH-to-total luminosity ratios increase with decreasing luminosity, but deeper data is required to confirm this result.

Comments: Accepted on The Astrophysical Journal on August 20th 2013, emulateapj, 14 pages, 16 figures
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1308.4683 [astro-ph.CO]

The WISE properties of complete samples of radio-loud AGN

G. Gurkan, M. J. Hardcastle, M. J. Jarvis
(Submitted on 22 Aug 2013)

We present an analysis of four complete samples of radio-loud AGN (3CRR, 2Jy, 6CE and 7CE) using near- and mid-IR data taken by the Wide-Field Infrared Survey Explorer (WISE). The combined sample consists of 79 quasars and 273 radio galaxies, and covers a redshift range 0.003<z<3.395. The dichotomy in the mid-IR properties of low- and high-excitation radio galaxies (LERGs – HERGs) is analysed for the first time using large complete samples. Our results demonstrate that a division in the accretion modes of LERGs and HERGs clearly stands out in the mid-IR-radio plane (L_(22 \mu m) = 5×10^(43) erg s^(-1)). This means that WISE data can be effectively used to diagnose accretion modes in radio-loud AGN. The mid-IR properties of all objects were analysed to test the unification between quasars and radio galaxies, consistent with earlier work and we argue that smooth torus models best reproduce the observation. Quasars are found to have higher mid-IR luminosities than radio galaxies. We also studied all the sources in the near-IR to gain insights into evolution of AGN host galaxies. A relation found between the near-IR luminosity and redshift, well-known in the near-IR, is apparent in the two near-IR WISE bands, supporting the idea that radio sources are hosted by massive elliptical galaxies that formed their stars at high redshifts and evolved passively thereafter. Evaluation of the positions of the sample objects in WISE colour-colour diagrams shows that widely used WISE colour cuts are not completely reliable in selecting AGN.

Comments: 13 pages, 10 figures, submitted to MNRAS
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)
Cite as: arXiv:1308.4843 [astro-ph.CO]
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